The present invention relates to wireless communication methods, base stations, and wireless communication systems. In particular, the present invention relates to a wireless communication method, a base station, and a wireless communication system in which a base station for performing cellular communication is an array-antenna wireless device provided with a plurality of antennas and generates beams in a time-division manner to perform packet transmission.
The antennas used in base stations of cellular systems are directional antennas for forming sectors; some of them do not include array antennas for segmenting the sectors further. Each base station uses the same frequency channel, and therefore, crosstalk with other base stations causes interference. The base stations transmit pilot signals, and a mobile station receives these signals and measures the respective signal levels. At the mobile station, it is possible to calculate the C/I ratio (the ratio of carrier power to interference power) from the measured signal level and to calculate a forward-link transmission data rate from the calculated C/I ratio. The calculated transmission rate is then wirelessly transmitted to the nearest base station. Then, based on this information, one of a plurality of modulators provided at the base station specifies the data rate selected by the mobile station to modulate user information coming from a network. The modulated signal is transmitted as a wireless signal from the antenna of the base station using the same radiation pattern (for example, a sector pattern) as the pilot signal.
One example of a cellular system is a CDMA2000 1xEV-DO system. Detailed specifications of this system are available in The Third Generation Partnership Project 2 (3GPP2) Specifications, C.S0024-v4.0, “cdma2000 High Rate Packet Data Air Interface Specification” (online document accessed on Aug. 29, 2005, URL: http://www.3gpp2.org/public_html/specs/tsgc.cfm). In this system, a pilot channel and a data channel transmitted from a base station are time-division multiplexed. In a mobile station, a forward-link transmission data rate is successively calculated from the C/I ratio of the time-division multiplexed pilot signal, and the data rate is successively requested from the base station. That document contains no specifications regarding formation of a radiation pattern by an array antenna and a method of determining the forward-link data rate in a system using such a pattern, nor do the other documents.
A method of determining the forward-link data rate of a base station using an array antenna in a CDMA2000 1xEV-DO system is disclosed in Japanese Unexamined Patent Application Publication No. 2003-304577. In the disclosed method, a plurality of base stations transmit pilot signals using array antennas. Mobile stations receive the pilot signals from the plurality of respective base stations, estimate their propagation paths, and transmit propagation path information to the base stations. The base stations estimate the forward-link reception quality using the propagation path information received from the mobile stations and determine the forward-link transmission rates to the mobile stations.
Another method is disclosed in Japanese Unexamined Patent Application Publication No. 2004-165834.
In this method, a base station transmits a reference signal (pilot signal) to a mobile station using an array antenna in a directional manner. The mobile station measures a signal-to-interference ratio (SIR; the ratio of desired signal to interference signal) from the received reference signal and transmits SIR to the base station. The base station applies adaptive modulation and encoding to packet data based on the received SIR. The base station transmits the modulated packet data to the mobile station with the same directivity as that used to transmit the reference signal; however, the directivity is not switched until transmitting the packet data.
Wireless communication systems which perform communication using narrow beam patterns for partitioning into sectors and which estimate the interference power from a pilot signal transmitted with a plurality of beam patterns to estimate the C/I ratios and obtain forward-link data rates have also been disclosed (for example, see Japanese Unexamined Patent Application Publication No. 2003-338803). Japanese Unexamined Patent Application Publication No. 2005-143148, which is a divisional application of Japanese Unexamined Patent Application Publication No. 2003-338803, discloses a similar technology.
Communication systems which scan a plurality of beam patterns, before determining the antenna pattern for transmitting data, and which select the best beam pattern for the terminal for transmission have also been disclosed (for example, see Japanese Unexamined Patent Application Publication No. Hei-11-252614).